1. Field of the Invention
This invention relates to a semiconductor device and its manufacturing method, especially to the semiconductor device with a piercing hole.
2. Description of the Related Art
A conventional semiconductor device is explained with reference to FIG. 14 by using an up-drain type MOS transistor with a trench configuration as an example.
An epitaxial layer 52 is formed, for example, on a semiconductor substrate 51 made of an N type silicon and a P type diffusion layer 53 (channel region CH) is formed on the surface of the epitaxial layer 52, as shown in FIG. 14. A trench 54 extending from the surface of the P type diffusion layer to the predetermined depth of the epitaxial layer is also formed. A conduction layer made of a poly-silicon film surrounded by an insulating film 55 is buried in the trench 54, configuring a gate electrode (G) 56.
Additionally, an N type source layer 57 adjacent to the insulating film 55 is disposed at the both side walls of the trench 54 on the surface of the epitaxial layer 52. A P type body layer 58 (BD) is disposed to bridge the two source layers adjacent to each other.
Also, a drain layer 59 made of N type impurity is formed to extend from the surface of the epitaxial layer 52 to the predetermined depth of the semiconductor substrate 51.
A source electrode (S) 60 made of, for example, aluminum (Al) alloy covering the source layer 57 and a drain electrode (D) 61 made of aluminum (Al) alloy covering the drain layer 59 are formed on the epitaxial layer 52.
A metal film 62 is disposed on the back surface of the semiconductor substrate 51, completing a semiconductor device 63.
The relevant technology is disclosed in the Japanese Patent Application Publication No. 2004-363302.
An electric current I2 goes through from the source electrode 60, the epitaxial layer 52, to the semiconductor 51, then goes through inside the metal film 62, and goes again through the semiconductor substrate 51 to the drain electrode 61 in an up-drain type MOS transistor with a trench mentioned above, along with the arrow shown in FIG. 14.
However, the resistance value of the semiconductor device becomes high since the electric current goes through the part of semiconductor substrate 51, where the resistance value is high because of the absence of the epitaxial layer 52, twice. Therefore, there is a problem of not being able to lower the resistance value of the semiconductor device.